The principle of valve-lift changeover on internal combustion engines is known from the prior art. Thus, systems such as Vario-Cam-Plus from Porsche have already been in series production for several years (see also “Handbuch Verbrennungsmotor”, Publisher: van Basshuysen/Schäfer, Vieweg Verlag, 1st edition 2002, chapter 10.4
The problem with this technology is still the lift changeover process itself. Difficulties arise with respect to the torque neutrality of the internal combustion engine precisely at the changeover from a small to a large valve lift. Compared with this, the changeover from a large to a small valve lift can preferably take place in the overrun fuel cut-off phases and is therefore inclined to be uncritical with regard to torque neutrality.
The valve-lift changeover frequently leads to jerky running of the motor vehicle. The reason for this is in the barely unavoidable inaccuracies in the model of the degree of aspiration or delivery (manifold model). These models calculate, in the engine management system, the air mass supplied to each cylinder. From this, the amount of fuel to be injected to maintain a specific air/fuel ratio (lambda) is calculated. The amount of fuel in turn determines the amount of engine torque produced for the particular combustion cycle.
A separate model is stored in the engine management system for each valve-lift configuration of the internal combustion engine. The model data is stored in maps formed from sampling points. Because the data content of these maps is usually identical for a variety of vehicles, for example from series production, the system individuality plays a considerable role in this case. Even small manufacturing tolerances can mean that two systems that are supposedly the same behave differently.
A measure of the quality of the valve-lift changeover is obtained by a comparison of the torque in both valve-lift configurations, i.e. directly before and directly after the valve-lift changeover. The torque for both valve lifts before and after the valve-lift changeover should be as equal as possible. Even small errors in the model calculation can lead to differences in torque and therefore to jerky running of the vehicle during the valve-lift changeover. Even though the cylinder charging models are usually adjusted by existing sensors, such as inlet manifold pressure sensors, air mass meters etc, these components are themselves subject to tolerances and therefore small inaccuracies cannot be ruled out.
This brings us to the cost-accuracy problem. At a realistic price it is possible to develop and manufacture a sensor with only a limited accuracy. A further difficulty is that the combustion behavior of the spark-ignition engine will in all probability alter due to the valve-lift changeover. This is to be expected mainly with ported injectors due to the changed flow cross section and changed flow conditions at the valve. A particular problem then arises if a change in the combustion process also takes place during the valve-lift changeover. This, for example, will be a change from a spark-ignition engine type combustion to a controlled spontaneous ignition.
In principle, the models can be translated in the engine management system with any required accuracy. The disadvantage, however, is the rapidly increasing complexity of the data processing sequences that arise when all the influencing parameters have to be captured with a specific accuracy on a variable to be mapped. Even modern engine management systems quickly reach their limits in this case. In particular, the storage and computing capacity of the engine management system is quickly exhausted, so that finally compromises have to be made and limits placed with regard to accuracy. A similar approach applies for the sensor systems used in the spark-ignition engine, because large series products demand inexpensive components for which a certain measuring tolerance has to be accepted.